Alkaline fuel cells (AFCs) are one of the most developed technologies and have been used since the mid-1960s by NASA in the Apollo and Space Shuttle programs. The fuel cells on board these spacecraft provided electrical power for on-board systems, as well as drinking water and were selected because they are among the most efficient in generating electricity having an efficiency reaching almost 70%.
The NASA AFCs used an aqueous electrolyte, specifically a solution of potassium hydroxide (KOH) retained in a porous stabilized matrix. The charge carrier for an AFC is the hydroxyl ion (OH−) that migrates from the cathode to the anode where they react with hydrogen to produce water and electrons. The water formed at the anode migrates back to the cathode to regenerate hydroxyl ions. The entire set of reactions is then:Anode Reaction: 2H2+4OH−=>4H2O+4e−Cathode Reaction: O2+2H2O+4e−=>4OH−Overall Net Reaction: 2H2+O2=>2H2O
Despite their high efficiency, reasonable operating temperatures and other positive attributes, the NASA AFCs were very sensitive to CO2 that is likely to be present in the fuel used by the cell or environmentally. This sensitivity comes from even trace amounts of CO2, CO, H2O and CH4 reacting with the KOH electrolyte, poisoning it rapidly, and severely degrading the fuel cell performance by either the dilution of the electrolyte or the formation of carbonates that reduce the electrolyte's pH and hence the kinetics of the electrochemical reactions at the level of the electrodes, impairing their performance. Therefore, such AFCs were limited to closed environments, such as space and undersea vehicles, running on pure hydrogen and oxygen can be worthwhile.
On the positive side, and in addition to their high efficiency and low operating temperature, AFCs are the cheapest fuel cells to manufacture as the catalyst that is required on the electrodes can be any of a number of different materials that are relatively inexpensive compared to the noble catalysts required for other types of fuel cells. Therefore, there has been considerable interest in solving their sensitivity to poisoning, in a manner other than providing pure or cleansed hydrogen and oxygen, and take advantage of the AFCs positive attributes such as operation at relatively low temperatures and high efficiency to provide a quick starting power source and high fuel efficiency, respectively.
In recent years, interest has grown in the development of anion exchange membranes (AEMs) for use in AFCs and electrolyzers due to the low overpotentials associated with many electrochemical reactions at high pH and the potential to forego noble metal catalysts. AEMs serve as an interesting counterpoint to the more widely developed and understood proton or cation exchange membranes (PEM or CEM). However, there are no readily available anion exchange membranes that serve as a commercial standard for electrochemical applications such as DuPont's Nafion® PSFA (perfluorosulfonic acid) membranes do in the field of cation exchange membranes.
The use of anionic fuel cells based on solid polymeric anion exchange membranes (AEMs) have been demonstrated and their use in both AFCs and alkaline-membrane DMFCs (direct methanol fuel cells). Further, the use metal-free anion exchange membranes operating at elevated pH potentially lowers or eliminates the need for noble metal based catalysts and improves the kinetics of the electrochemical reactions. There are additional advantages to AEMs, and in particular to polymeric alkali anion exchange membranes (AAEMs). For example, it is likely that electrode construction and orientation limitations can be overcome for AAEMs as the conducting species would be incorporated into the fixed solid polymer AAEM. Additionally, even though some CO3-2/HCO3— formation at the anode is likely to occur, there are no mobile cations (Na+ or K+) present in the AAEM to precipitate solid crystals of metal carbonate to block or destroy the electrode layers since with AAEMs the cations are immobilized. Further, as there is no liquid caustic electrolyte present, electrode weeping and component corrosion should be minimized.
Therefore there is a need for AAEMs that have the necessary conductivity, resistance to water swelling, mechanical strength, and chemical stability at operating temperatures to provide the next generation of AFCs.